How vulnerable are bryophytes to climate change? Developing new species and community vulnerability indices

Species' vulnerability to climate change is often assessed by focusing on potential changes of species' ranges. This study aimed to develop community-level vulnerability indices which measure bryophyte community vulnera-bility to climate change, based on the best set of factors summarizing species' niche or geographic properties expected to respond to climate change. We used a dataset on 39 saxicolous bryophytes from the Iberian Peninsula, highly sensitive to climate shifts. Niche metrics were calculated using a recently described hypervolume-based approach. Spatial metrics were derived from habitat suitability model (HSM) projections. We then compared regression models based on niche or spatial metrics to evaluate which ones improve species range shifts forecast. The final vulnerability score for each species, the Species Vulnerability Index (SVI), was calculated by applying a weighted sum of all the relevant parameters. We then generated a spatial representation of vulnerability values for the whole community through HSMs and obtained three Community Vulnerability Indices (CVIs), according to different statistical aggregation measures (average, maximum and standard deviation). SVI assigns maximum vulnerability to species with smaller niche breadth and higher marginality in the community environmental niche space continuum, allowing to rank bryophyte species according to their vulnerability. Given the overall importance of niche-hypervolume metrics in SVI and CVIs, we rename it, respectively, as Niche Hypervolume Species Vulnerability Index (NHSVI) and Niche Hypervolume Community Vulnerability Indices (NHCVIs). Overall, saxicolous bryophyte communities with the greatest average vulnerability to climate change are those at the high mountains of the northern, central and southern regions of the Iberian Peninsula. Results suggest that vulnerability patterns are structured locally not only due to species richness but also to community composition. The three NHCVIs provided complementary insights into the study area's community vulnerability distribution. This study shows that NHSVI can prioritise vulnerable species to climate change, and NHCVIs can depict community-wise vulnerability hotspots, thereby informing policymakers in the definition of bryophyte species conservation measures.